Low temperature heat exchanger



Oct. 29, 1968 G. KLIPPING LOW TEMPERATURE HEAT EXCHANGER Filed sept.14,v 196e INV ENIOR au u Gustav Klpping ATTORNEYS United States Patent Ov 3,407,615 LOW TEMPERATURE HEAT EXCHANGER Gustav Klipping,Berlin-Zehlendorf, Germany, assigner to Max-Planck-Gesellscllaft zurForderung der Wissenschaften eN., Gottingen, Germany Filed Sept. 14,1966, Ser. No. 579,244 Claims priority, application Germany, Sept. 14,1965, M 66,623 4 Claims. (Cl. 62-45) ABSTRACT F THE DISCLOSURE A heatexchanger for use in low temperature heat exchange devices using lowboiling fluids and, particularly, helium, hydrogen and nitrogen. Theheat exchanger has a housing whose walls are shrink-fitted onto asintered metal insert, and is therefore characterized by an extremelyhigh heat conductivity.

The present invention relates generally to the cryogenic field, and,more particularly, to a heat exchanger which uses the low temperature oflow boiling refrigerants, and particularly helium, hydrogen, andnitrogen, which are fed in the liquid phase to the interior of the heatexchanger in which a sintered metal insert or charge is disposed forimproving heat transfer.

In known continuous flow cryostats for producing low temperature, thelow boiling refrigerant is fed by a vacuum pump into the vaporizingchamber of the cooling head Where it evaporates and brings about thecooling of the cooling head which is provided, for example, with a probeholder, and thus cools the probe which is supported thereon. For manyapplications such devices are required to have a high heat load at lowtemperatures of down to 2.5" K. As a prerequisite for a high heat load,however, the heat exchange between the refrigerant and the probe holdermust be as complete as possible. Previously known heat exchangers wereprovided as hollow chambers, and in order to improve the heat transferbattles or fillers, such as sintered metal bodies, were introducedtherein. Gther heat exchangers were provided as a spiral pipe or as asolid body having flow chanels and it was sought to provide as ygood aheat transfer as possible by branching off and providingcountercurrently directed pipes and channels.

With this in mind, it is a main object of the present invention toprovide a heat exchanger for low boiling refrigerants havingconsiderably greater heat load at low temperature than previous devices.

Another object of the present invention is to provide a heat exchangerincluding a sintered metal which has an extremely good heat conductingconnection between sintered metal and the body which supports it.

A further object of the present invention is to provide a heat exchangerin which a housing is shrunk onto a sintered metal insert to providegood heat exchange contact between the insert and the housing.

These objects and others ancillary thereto are accomplished inaccordance with preferred embodiments of the invention wherein the heatexchanger is shrunk with its wall portions onto a sintered metal insertor charge within the area of its circumference. In accordance with afurther feature of the invention chambers are provided both in front ofand to the rear of the sintered metal insert or charge and thesechambers have cross-sectional areas which correspond approximately tothe free surface of the sintered metal insert or charge.

It may be particularly advantageous in this arrangement to provide thehousing of the heat exchanger to have a pot-like construction. The freeopening receives the rce cylindrical sintered metal insert or charge andthe supply line for the refrigerant terminates at a closing element ofthe heat exchanger which closes off the free opening of the pot-likemember.

Additional objects and advantages of the present invention will becomeapparent upon consideration of the following description when taken inconjunction with the accompanying drawings in which:

FIGURE 1 is a schematic cross-sectional view through a sintered metalheat exchanger for cooling the probe.

FIGURE 2 is a schematic view of a sintered metal heat exchanger in acontinuous ow cryostat disposed on the refrigerant storage tank.

As can be seen in FIGURE l a pot-shaped housing 2 which may be of copper-is shrunk onto a preformed sintered metal insert or charge which may beof copper too to get an extremely good heat conductive connection withthe housing 2 over the entire periphery or circumference.

A sintered metal, eg., copper, brass, silver or the like, is a porousmaterial which is made by the sintering of a powder of spherical orspattered particles of the same size. The porosity can be Widely variedby variations of sintering temperature and sintering time. A sinteredmetal is machinable as a compact metal and can be soldered or weldedlike a compact material.

Shrinking the housing 2 onto the sintered metal insert 1 lrneans thatthe housing with a bore of given diameter is warmed up to about C.whereby it is expanded. At the same time the sintered metal insert whichhas a diameter slightly bigger than the diameter of the bore inside thehousing is cooled to about -200 C. (liquid nitrogen) whereby it isshrunk to a diameter which is smaller than the diameter of the enlargedbore inside the warm housing. Both parts are then tted into each other.When the combined parts have reached room tem perature, the molecularforces operating between the housing and the sintered metal insert willprovide an optimum connection which is similar to an intermetalliccompound or a solid solution, and which results in optimum heatconduction between both parts shrunk together. The housing will thus bein a. shrinlotted relationship to the insert. The difference between thediameters of both parts in the initial separated state at roomtemperature, which was described above as having the sintered metalinsert slightly bigger depends on the materials of both parts and theirexpansion coeicients, on the diameter, and on the temperature differencethat can be achieved before the parts are fitted into each other.Preferably the insert and the housing have approximately the samecoeicient of thermal expansion.

As FIGURE l shows the pot-shaped housing 2 is closed off at its freeopening thereof by a closure member or cover 3 and a refrigerant supplyline 4 is disposed in cover 3. An exhaust gas line 5 is connected to theupper end of the housing 2. The housing 2 is provided as a probe holderin which the probe 6 is held in place by means of a dip solderedconnection 7. A temperature sensor 8 is disposed in the housing 2 inproximity to the probe 6. In the ernbodiment illustrated the temperaturesensor 8 is connected as part of a vapor pressure thermometer, but it isequally possible to use instead electrical sensors or both types ofsensors.

During operation the refrigerant enters from the supply line 4 into thedistributing chamber 9 for the refrigerant and fiows from there to theinterior of 'the sintered metal insert or charge 1. Here, heat exchangetakes place While the refrigerant evaporates, and possibly the cold gastoo absorbs some heat. The resultant cold gas flows into the collectorchamber 10 and from there into the exhaust gas line 5. The heat which isto be conducted away from the probe 6 is conducted through the housing2, which must be suitably designed, and is conducted through thesintered metal insert or charge 1 to the refrigerant. A suitablydesigned housing means that the wall thickness must be sufficient enoughso that the maximum heat load can flow through this wall to the sinteredmetal, i.e., the necessary wall thickness depends on the material of thehousing and its coeicients of heat conduction at low temperatures and onthe maximum heat load. For better understanding a numerical example willbe given: for a housing made of copper (coeicient of heat conduction at42 K. is \=4W/cm.2 K.), a height of the housing of 4 cm., a temperaturedifference between inside and outside surface of 1 and a heat load of10W, the wall thickness should be about 1 cm.

FIGURE 2 shows a sintered metal heat exchanger 11 disposed in thecooling head 12 of a continuous flow cryostat for probe cooling. In aknown manner the cooling head is disposed in a vacuum housing 13 andcornmunicates with a refrigerant storage tank 15 by means of riser 14.It also communicates via exhaust gas line 16 which is arranged in theform of a radiation shield 16a by being coiled around the cooling head.It is provided with a regulating valve 18 which is thermostaticallycontrolled in dependence upon the evaporator temperature and this isaccomplished by means of a temperature sensor 17. It is also connectedwith a feed pump 19. This could be used with the apparatus disclosed inU.S. Patent No. 3,166,915.

The heat transfer which may be accomplished in a heat exchanger dependsamong other things upon the size of the exchange surface. In order toobtain a high heat load of the heat exchanger the exchange surfaceShould be as large as possible. Sintered metals have a large specificsurface due to their porosity and, since they are made from the metalscopper, silver, brass and the like, they also have relatively good heatconduction. Also, even with the smallest pore width or diameter theyhave sufcient permeability for the low boiling refrigerants due to thelow viscosity of the latter. Moreover, particularly good heat transfercan take place between low boiling uids and a metal within a sinteredmetal body. On the other hand the quality of the heat exchange betweenthe refrigerant and the probe which is supported at the surface of theheat exchanger depends to a large extent upon the heat transfer betweenthe sintered metal insert or charge and the housing of the heatexchanger which encloses it. The sintered metal body which itself hasrelatively good heat conduction must therefore be brought into a goodconducting connection with the cooling head which is the probe holder.It is particularly advantageous to form the sintered metal body, whichmay be machined as a solid material, true to size or measure on a partof the surface thereof and to shrink the cooling head onto it, becausethere results a connection with optimum heat conduction characteristics.Advantageously according to this method the pores at the free surface ofthe sintered metal body remain intact, i.e., the free surface remainsfully permeable for the refrigerant. Thus, the heat exchanger provided,as proposed by the present invention has a considerably greater heatload at low temperatures than do the known devices.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

What is claimed is:

1. A heat exchanger for use in low temperature heat exchange devicesusing low boiling uids and, particularly, helium, hydrogen and nitrogen,comprising, in combination, a heat exchanger housing having walls; and apreformed sintered metal insert disposed within said housing and formingtherewith an intermetallic compound or a solid solution; said housingbeing in shrink-fitted relationship to said insert, thereby providing anelement having an extremely high heat conductivity.

2. A heat exchanger as dened in claim 1 wherein in the direction of theiiow of refrigerant a distributing chamber is disposed before thesintered metal insert in the housing and a collection chamber isdisposed after the sintered metal insert in the housing, thecross-sectional surfaces of said chambers corresponding approximately tothe free surface of the sintered metal insert.

3. A heat exchanger as dened in claim 2 wherein the housing ispot-shaped and in its free opening receives the sintered metal insertwhich is cylindrical, a cover closing olf the free opening of saidhousing, and a refrigerant supply line being connected to said cover soas to communicate with the distribution chamber.

4. A continuous flow cryostat with a cooling head having a heatexchanger as defined in claim 1.

References Cited UNITED STATES PATENTS 2,267,339 12/1941 Paulsen 29-4472,448,315 s/1948 Kunzog -154 x 2,663,626 12/1953 Spangler 62-48 X3,302,415 2/1967 Royer 62-48 x ROBERT A. OLEARY, Primary Examiner.

